This invention relates to an all metal seal for demountable joints with flanges which provides leak tight sealing over a range of temperatures from liquid helium temperatures upwards to temperatures approaching the melting point of the sealing agent.
The four basic types of seals used in demountable joints are O-rings, C-rings, gaskets and compression fittings. These seals are used in a wide variety of applications including rubber gaskets for "Mason" jars and "Viton" O-rings for the solid fuel rocket boosters of the space shuttle. While useful for many applications, the known seals are generally inadequate for sealing fluid systems at cryogenic temperatures or at temperatures above the working range of elastomer materials.
Cryogenic fluid systems are used extensively in high energy physics research and, generally, helium is used as the working fluid. Due to the small atomic size of helium, however, it is an extremely difficult fluid to seal. To provide adequate thermal insulation, cryogenic systems are often vacuum insulated; and very small leaks, which in an ambient pressure environment are of no consequence, can spoil the vacuum. Because the known seals for demountable joints are not totally effective in sealing cryogenic working fluids such as helium, the piping joints in cryogenic systems are oftentimes welded or soldered. Soldered joints and welded joints in cryogenic systems have several drawbacks, however. Welding may damage heat sensitive components such as diode temperature sensors and soldering may introduce contaminants into the system which tend to freeze-out in small flow passages and cause blockage thereof. In addition, welded or soldered joints effectively eliminate the possibility of easily removing system components for maintenance or testing.
For the reasons stated, it is desirable to use demountable joints in cryogenic systems. To this end, it has been recognized that soft ductile metals are the best sealant materials for demountable joints for cryogenic service below liquid nitrogen temperatures. Indium metal is used as the sealing agent in many of the known demountable joint designs. Indium is advantageous in that it remains soft and ductile at cryogenic temperatures and flows easily into irregularities in the surfaces being sealed, thereby forming a vacuum-tight seal. Indium, however, is disadvantageous in that because it is soft and ductile it is easily extruded from between the surfaces being sealed, thus allowing them to leak. This extrusion may occur during thermal cycling when the seals are successively cooled and heated between cryogenic and room temperature, or it may be caused by joint vibration.
Various techniques have been proposed to mitigate the problem of sealant extrusion. Among these are O-ring grooves and precoating the mating surfaces with the gasket material. The proposed techniques have only met with limited success, however, and therefore, the need still exists for a reliable, demountable, cryogenic seal which can withstand repeated cycling from room temperature to cryogenic temperatures as well as significant bending forces without leaking.
U.S. Pat. Nos. 1782,014, 2,249,127, 2,327,837, and 4,418,928 have been located; however, no representation is made that they are relevant prior art or that they are the only prior art to this invention. Rimmelspacher, U.S. Pat. No. 1,782,014, discloses a packing gasket which has for its primary purpose providing an improved gasket construction for sealing joints in pumps. The disclosure indicates that the gasket portion of the body is made of cork and is enclosed in a sheet metal casing. As will be appreciated, cork would not work effectively as the sealant material in a cryogenic fluid system. In Goetze, U.S. Pat. No. 2,249,127, there is disclosed a composite gasket consisting of a pair of packing elements disposed within a sheet metal casing or shell. The disclosed packing elements are made of asbestos or an asbestos compound, and therefore would not work effectively as the sealant material in a cryogenic fluid system. Williams, U.S. Pat. No. 2,327,837, discloses an S-shaped retainer configuration for a seal-gasket, however, it discloses using a packing material of cement and asbestos. Again, it will be appreciated that because of its porous nature such a packing material is wholly unsuited for use in cryogenic fluid systems. Fontana, U.S. Pat. No. 4,383,694, discloses a gasket device for statically sealing high pressure and temperature fluids. The gasket device of Fontana comprises an S-shaped metal liner which defines two cavities that contain inserts of an elastic sealant material. Several materials are disclosed for use as the sealing inserts in Fontana; for example, rubber, vegetal fibers, Teflon, reinforced rubber, asbestos filaments, compressed graphite-asbestos, and other non-metallic materials. None of the disclosed sealant materials would appear to be effective as a long term sealant in a cryogenic fluid system wherein helium is the cryogenic fluid. Finally, Nicoll, U.S. Pat. No. 4,418,918, discloses using an indium alloy as the sealant material in a threaded cryogenic seal having opposed annular recesses.